1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device (hereinafter, referred to simply to as an “organic EL device”). More particularly, the present invention relates to a white light-emitting organic EL device and a method for fabricating the device.
2. Discussion of the Related Art
In general, organic electroluminescent (EL) devices consist of: an anode made of materials, e.g., indium tin oxide (ITO); a cathode made of materials, e.g., aluminum (Al); and functional organic layers sequentially laminated between the anode and the cathode. Upon application of an electric field, organic EL devices emit light. Organic EL devices have advantages in that they can be operated at a relatively low voltage and can be fabricated on flexible transparent substrates. Other advantages of organic EL devices are relatively low power consumption and light-weight.
In particular, research on white light-emitting organic EL devices has been conducted since the end of 1980s. Since white light-emitting organic EL devices can be widely utilized in applications including organic EL source and organic EL illumination, they have been the focus of intense interest lately.
There has been actively developed a variety of methods fabricating for white light-emitting organic EL devices that satisfy the requirements of high color purity, superior color stability, superior optical properties depending upon an electric current, excellent luminous efficiency, long lifetime, and easy fabrication.
A variety of methods for realizing light-emission of white light-emitting organic EL devices will be described.
FIG. 1A illustrates monolayer light-emission realizing white light-emission via a single light-emitting layer. FIG. 1B illustrates multilayer light-emission realizing white light-emission via perpendicular combination of respective colors emitted from a plurality of light-emitting layers. FIG. 1C illustrates color conversion to which a blue light-emitting layer and a fluorescent layer are applied. In the color conversion, light emitted from a blue light-emitting layer undergoes an emission-color change from blue to red via fluorescent radiation derived from a fluorescent layer, thus realizing white light-emission via combination of the blue and red light. FIG. 1D illustrates a microcavity structure utilizing wavelength variation via a plurality of color conversion layers. FIG. 1D illustrates a laminated structure comprising red, green and blue light-emitting organic EL device units.
The multilayer light-emission is most commonly used among these methods.
FIG. 2 is a cross-sectional view illustrating a conventional multilayer light-emission structure of a white light-emitting organic EL device. As shown in FIG. 2, the conventional white light-emitting organic EL device has a structure where a first light-emitting layer and a second light-emitting layer are interposed between an anode and a cathode.
The white light-emitting organic EL device having the multilayer light-emission structure is required to stabilize the interface between adjacent layers (in particular, between adjacent light-emitting layers) to realize the desired CIE chromaticity coordinates for white light and improve luminescence efficiency and lifetime.
That is, the electric charge balance and distribution in the light-emitting layers considerably affect white representation.
In particular, in a case of where heat-treatment involves, it is more important to prevent variation in color representation caused by diffusion of atoms and molecules inside the light-emitting layers.
Accordingly, there has been repeated research to improve color representation, life time and efficiency of white light-emitting organic EL devices. However, substantially no satisfactory results have been achieved. The lifetime and efficiency of organic EL devices still remain a problem.